The term "surface filter" means here a plane surface filter with reduced thickness, or with a limited effective filter depth, independent of the manner the filtering material is arranged. The individual effective filter surfaces have then a relatively small distance to the boundary of a filter assembly or a filter body, or element. In principle, filter plates could be used. However, as a rule the filter is formed by filter strips, made of paper, synthetic material or other such thin filter material, which can be arranged in a zig-zag manner or wave-like manner, in order to increase the effective filter surface obstructing access for cleaning purposes.
For example, two filter plates are arranged slantedly over an inner upper edge of the coating zone, according to U.S. Pat. No. 4,303,417, wherein the remaining space, defined by this division, becomes the clean gas zone, from which the filtered air is exhausted. However, in this case the available filter surface is just as limited as the qualitative and quantitative filter efficiency and the replacement of the high-positioned plate-like filter elements is difficult. When the color is changed, and also in the case of each powder change, the filter plate has to be taken out and cleaned outside the booth.
A self-acting cleaning device for filter elements is also shown in German published specification No. 25 50 918, wherein a horizontally slidable filter carriage is used for the cleaning of vertically arranged and mutually parallel filter pockets which are subjected to periodic pulses of compressed air.
Another self-acting cleaning device is shown in the German published specification No. 14 07 293, according to which horizontally arranged blow pipes with several nozzle openings are slidable along the surface of screens or fan filters arranged in a V for dust collector devices. Neither device is suitable for powder recovery devices in spray booths.
Further, a coating booth is known from German published specification No. 19 51 361, wherein a clean gas zone is separated from its back wall by a vertically arranged filter, folded in a zig-zag pattern. In this case the filter folding of the thin filter material is so coarse, that the total thickness amounts to more than 15% of the entire booth depth. As a result, the filter can be only roughly cleaned and has to be dismounted at each color change. In addition it is very unstable and can be exposed only to reduced pressure loads.
An even more coarsely folded zig-zag filter, whose thickness amounts to about half of the booth depth and which is mounted on a removable carriage on the rear side of the booth, is shown in U.S. Pat. No. 4,401,445. For the cleaning, the filter carriage has to be removed entirely from the booth. In this case, the cleaning operation is also cumbersome and mostly incomplete.